1. Field of the Invention
The invention relates to a heavy vehicle such as a transport vehicle or a “civil engineering” vehicle, of a mass greater than 500 metric tons and fitted with tires of a diameter greater than three meters fifty having an axial width greater than 37 inches.
2. Description of the Related Art
Such vehicles, generally designed for carrying heavy loads, comprise a front steering axle comprising two steering wheels and a rear axle, which is most frequently rigid, comprising four driving wheels distributed in pairs on each side.
An axle is defined as an assembly of the elements enabling the fixed structure of the vehicle to be connected to the ground.
The axial or transverse direction of the tire is parallel to the axis of rotation of the tire.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the rolling direction of the tire.
The axis of rotation of the tire is the axis around which it turns in normal use.
In the case of vehicles, in particular intended for use in mines or quarries for transporting loads, the problems of access and demands of production lead the manufacturers of these vehicles to increase their loading capacity. It follows that the vehicles become ever larger and thus ever heavier in themselves and are capable of transporting an ever greater load. The current masses of these vehicles may reach several hundred metric tons, the same being true of the load to be transported; the total mass may reach 600 metric tons.
At present, as stated above, vehicles of this type, such as dumpers used in mines, comprise a rear driving axle on which are mounted four wheels, arranged in pairs, to respond to these demands.
Furthermore, the dimensions of said wheels and consequently those of the tires and in particular the rigidity of the lower zones require said wheels to be made in several parts to allow mounting of the tire on a rim. The operations involved in mounting and demounting said tires in the event of replacement or maintenance are long and tedious. The number of clamping parts which have to be handled during these operations may be greater than 200, and very high levels of clamping torque are associated therewith. The time taken for these operations is consequently long and therefore detrimental to the productivity sought during exploitation of said mines.
Since current demands tend towards a constant increase in the loading capacity of these vehicles, the various parameters listed above have resulted in widening of the tires so as to increase the volume of air therein. At around 4 meters, tire diameter is now such that it is virtually impossible to increase it further, in particular for reasons of transportation of said tires. In fact, the dimensions of said tires will be limited by transportation thereof, in particular by road widths and bridge headroom. It is likewise virtually impossible to decrease the rim diameter, which in particular allows positioning of the driving torque transmission system and of the braking systems.
During their studies, the inventors have succeeded in demonstrating that these “widened” tires do effectively allow an increase in transported load but exhibit various drawbacks. Tests have in fact shown that the wear resistance of these tires is reduced, in particular with respect to the axially outer tires mounted on the rear axle, due in particular to torque phenomena induced between the tires in particular on bends. More frequent tire changes reduce the efficiency of said vehicles. Current vehicles are equipped with differential gears between the wheels of one and the same axle disposed either side of an axis of symmetry of said axle, to limit said phenomena.
Furthermore, it has been demonstrated that, under certain travel conditions with a certain load, handling of the vehicle, in particular over curved trajectories, is greatly reduced. In fact, when the tires of the front axle are turned, in particular when the vehicle is loaded and for passing round bends with relatively small radii of curvature, it is possible that the vehicle may follow a substantially rectilinear trajectory. It would appear, in fact, that under certain loading and travel conditions, handling of such a vehicle over a curved trajectory, when fitted with four tires such as described above on the rear axle, is very difficult or indeed impossible, since the vehicle does not respond to the turning imposed by the tires of the front axle. These conditions may furthermore result in splitting and destruction of the tires of the front axle and thus once again in more frequent tire changes.
Furthermore, patent application WO 00/71365 describes a method making it possible to simplify tire mounting, the latter being mounted directly on the hub, which serves as the rim. Independent rings then act as rim seats and are held in place by locking rings, which are connected firmly to the hub as a result in particular of being complementary in profile.